Help with some diagnosis

[rickware]

Hope someone out there might be able to help me.
I have a 944 turbo 91, and it seems to develop a massive misfire / boost cut when hot and you get to the high rev range. It makes boost fine when cold and warm but after a few miles of hard driving startS to mis fire and cut out after 4000 rpm. seems like the boost is cutting out as it produces loads of black smoke as if it is over fueling.

my diagnosis was a boost leak or faulty temp sender but i have just done a boost test and it holds 1 bar pressure pretty well. (slowly leaks down to 0 but it takes a good few minutes to leak out and i can't hear any obvious leaks.)

i noticed though that the vacum pipe that goes from the bottom of the throttle body to the temprature valve at the back of the engine is missing. it seems the temp valve should have 2 pipes connected to it and the one to the throttle sensor has been snapped off at some point so i will have to buy a new valve to connect it. i don't suppose this is likely to be my problem but i had a new hybrid turbo fitted and then sent for a remap just before the problem started. I was wondering if it has been remapped with this vacum not connected whether this could be my problem?

i will also change the blue engine temp at the front as well as the valve to rule them both out.

any thoughts would be welcome

 

[peanut]

worth doing a search on this because I seem to remember that another member had exactly the same problem a few weeks ago and the garage found the same vacumn pipe disconnected.

 

[rickware]

thanks, i have found that the thermo valve opens at 58 deg to let unwanted fuel vapour escape and i think send it to a charcol filter? seems to me like i can just bypass it and connect the two together as i can't find the part for sale anywhere! i will search as you suggest though

 

[peanut]

i

i noticed though that the vacum pipe that goes from the bottom of the throttle body to the temprature valve at the back of the engine is missing. it seems the temp valve should have 2 pipes connected to it and the one to the throttle sensor has been snapped off at some point so i will have to buy a new valve to connect it.

I would say thats your main problem right there

 

[barks944]

Could be overboosting if the pressure in the vacuum line is too low to open up the wastegate to regulate boost. Might be the knock detection kicking in and retarding ignition that is causing the loss of power. You might get away with it when the engine isn't too warm as the boost will drop off naturally on standard turbos. 4k is probably about the point where you will generate peak boost before the boost pressure starts to tail off. There is some margin of safety in the standard car so it could be that you dont get knock until your head and valves are nice and hot.

Can you hook up a decent aftermarket boost guage so you can see exactly whats happening.

 

[rickware]

Thanks and sounds good advice. I am now putting it down to a change of AFM as when I remapped it I have since put a new AFM on. I still have the orginal chip so I will go back to this and test to see if this cures it. If it does then I'll have to pay for a new remap to make the most of the turbo. If I go back to the old chip and keep boost to under 0.9 bar, does anyone know if this will keep the engine 'safe' if I give it a good thrash to test? My turbo that I upgraded to is more efficient hybrid which is an AET K24 so I am slightly worried about the engine running to lean in standard form at the high revs.

Also, I checked the my new remap chip had the over boost override removed which it does, so I may now get the problem described above if I revert back to the old chip!